As a vaporization filter to be used in a humidifier, there is a material in which a part of a sheet state vaporization filter having a water-absorptive property is dipped in a water bath, water is subjected to suction by capillary phenomenon, and dry air is passed through to obtain humidified air (for example, see Patent Literature 1). However, in the vaporization filter with the system in which water is subjected to suction by capillary phenomenon, a minor amount of compounds containing an element such as silicon, calcium, magnesium, etc., which are contained in the supplied water causes scales and they attach to the vaporization filter, then, the effects of absorbing and raising water are decreased so that there is a problem that humidifying characteristics are lowered.
To solve such problems, there is disclosed a vaporization filter which comprises a foamy shaped substrate having a low water-absorptive property and a hydrophilic inorganic compound carried thereon, so that even when scales are attached, the humidifying characteristics are difficulty lowered (for example, see Patent Literature 2). However, even in the vaporization filter of Patent Literature 2, when it is used for a long period of time, lowering in humidification efficiency cannot be avoided due to attachment of scales to the inside of the foamy shaped substrate or voids at the surface layer, and no consideration has been done about long term durability in view of the humidification efficiency.
There has been also proposed a vaporization filter by providing voids with 10 to 25% using particles at the surface layer portion of a filter substrate having low water-absorption property, whereby the humidification efficiency are improved by maintaining water at the voids (for example, see Patent Literature 3). In the vaporization filter of Patent Literature 3, as the filter substrate, a foamy shaped (three-dimensional network structure) substrate has been used as in Patent Literature 2. Also, the surface layer portion having voids has a thickness of 10 μm or more, and the voids have a larger size of 1 to 3 μm2. Thus, with regard to the vaporization filter of Patent Literature 3, when it is used for a long period of time, it is difficult to avoid lowering in humidification efficiency due to attachment of scales to the voids at the surface layer portion, and no consideration has been done about long term durability in view of the humidification efficiency.    [Patent Literature 1] JP Patent No. 2514145B    [Patent Literature 2] JP 2005-315554A    [Patent Literature 3] JP 2007-198685A